The present invention relates to the dispersion of carbonaceous materials and more particularly to coal-aqueous coal mixtures.
Coal as an energy source is in abundant supply. It is estimated that in the United States there is more energy available in coal than in petroleum, natural gas, oil shale and tar sands combined. The substitution of coal for natural gas and oil on a large scale would therefore seem a ready-made solution to our energy problems. Unfortunately, however, unlike oil and gas consumption, coal use is limited not by reserves or production capacity but rather by the extraordinary industrial and regulatory difficulties of burning it in a convenient, efficient and environmentally acceptable manner.
A number of techniques are being explored to provide coal as a more useful energy source. One such technique employs gasification methods such as destructive distillation, to effect the conversion of coal to a low or medium Btu gas. In another approach, high pressure hydrogenation is utilized to liquefy coal to make it more suited for transport, burning and the like.
Another technique suggested, and the one to which the present invention relates, is the technique whereby solid coal particles are dispersed in a fluid carrier medium, such as fuel oil or water to form coal-aqueous or coal-oil mixtures.
Coal-oil and coal-aqueous mixtures, however, are distinct systems, each having its own difficulties of formulation. For example, while coal and oil are relatively compatible, coal and water are not. Thus, unlike in the formulation of coal-oil admixtures, in the formulation of coal-aqueous admixtures, the initial dispersing of the coal in the continuous water phase, especially large amounts of coal, represents a challenging obstacle. Moreover, after dispersion, stabilizing, i.e. keeping the coal from settling out of the water phase, must be also achieved.
Such coal mixtures offer considerable advantages. They are more readily transported then dry solid coal, are more easily stored and are less subject to the risks of explosion by spontaneous ignition, the latter being a significant factor in handling coal. In addition, providing coal in a fluid form can permit its burning in apparatus normally used for burning fuel oil. This can greatly facilitate the transition from fuel oil to coal as a primary energy source, another highly desirable result.
Various coal-oil and coal-aqueous mixtures have been described in the literature. For example, British Pat. No. 1,523,193 discloses a mixture comprised of fuel oil and from 15 to 55% by weight of finely ground coal particles reduced in particle size to 10 microns or finer. The use of fuel oil as a carrier medium negates the requirement of lessening our dependence upon fuel oil.
In U.S. Pat. No. 3,762,887, there is disclosed a dispersion of coal in an aqueous medium wherein the coal is ground to a defined array of particle sizes, a substantial portion of which being about 325 mesh Tyler Standard screen or even finer.
The article titled "Development and Evaluation of Highly-Loaded Coal Slurries" published in the 2nd International Symposium on Coal-Oil Mixture Combustion, Nov. 27-29, 1979, teaches coal-aqueous mixtures using coal of bimodal particle size distributions and containing modified starches, biocides and a wetting agent such as TRITON X, an octylphenoxy (ethyleneoxy) ethanol surfactant of low molecular weight.
British patent application GB 2 099 451A discloses aqueous coal suspensions which contain two separate groups of coal particles, the particles of the first group having an average size of from 210 to 60 .mu.m, the maximum size not exceeding 300 .mu.m and the particles of the second group having an average size of from 1/6 to 1/20 of the average size of the partices of the first group.
While the art has attempted to provide coal in dispersed fluid form, as evidenced by the above-described procedures, there still remains the need for improving these methods in order to provide higher solids and more stable coal mixtures. It would be highly desirable to provide coal in aqueous mixture form wherein only minor amounts of additive materials are needed to disperse the coal to high solids concentrations of 70% by weight, or higher. It would be further desirable to provide coal-aqueous mixtures wherein the coal is precleaned of impurities so that the resultant mixtures are clean burning or relatively clean burning and thus more environmentally acceptable.